Apparatus for heating and cooling semiconductor wafers in semiconductor wafer processing equipment

ABSTRACT

A method for heating or cooling a semiconductor wafer in semiconductor processing apparatus is described which comprises directing into contact with a surface of the wafer at least a portion of one or more components of the process gas to transfer heat between the wafer and a wafer support positioned in the apparatus adjacent to the wafer. Method and apparatus are also described for controlling the total flow of process gas through the apparatus and for monitoring the pressure in said apparatus to maintain the desired pressure therein.

This is a continuation of U.S. application Ser. No. 07/619,268 filedNov. 28, 1990, now abandoned, which is a division of U.S. applicationSer. No. 07/348,527 filed May 8, 1989, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods and apparatus for cooling and heatingsemiconductor wafers during the production of integrated circuitstructures. More particularly, this invention relates to wafer coolingand heating by means of a heat transfer gas.

2. Description of the Related Art

During the plasma etching of a semiconductor wafer, such as a siliconwafer, it is desirable to cool the wafer to avoid excessive heat buildupwhich can interfere with the processing. For example, excess heat cancause reticulation of the photoresist resulting in reduced sensitivityof the process. Other integrated circuit production processes such asChemical Vapor Deposition (CVD) require that the wafer be heated tosupport the proper chemical deposition reactions.

Wafer cooling is conventionally accomplished using a silicone rubberheat transfer pad which is mounted between the wafer and a pedestal andwhich functions as a wafer support and heat sink. However, it has beenfound that the transfer of heat from wafer to the pedestal heat sink viathe silicone rubber pad is not always sufficient to dissipate the amountof heat generated during the plasma etching process. Tests and analysisindicate that the primary heat transfer mechanism is due to thermalconduction of gas rather than due to contact conduction from the waferto the pedestal through the silicone rubber pad.

The use of gas as a thermal conductor in semiconductor wafer process isknown for transmitting heat either to or from a wafer. For example, KingU.S. Pat. No. 4,261,762 discloses a method for cooling a wafer beingbombarded by ions in an ion implantation apparatus wherein a wafer isclamped adjacent its periphery to a target block and a cooling gas witha high thermal conductivity such as nitrogen, neon, helium, or hydrogenis fed under pressure into the space between the wafer and the coolingblock through an orifice in the cooling block.

Lamont U.S. Pat. Nos. 4,680,061 and 4,743,570 describe the use of a gasto conduct heat to or from a wafer from a heat exchange means which maycomprise a heater for heating the wafer or a heat sink for cooling thewafer. A pressure plate seals the periphery of a wafer carrier plateassembly against the wall of a vacuum apparatus while the wafer iseither heated or cooled by the heat exchange means utilizing gasconduction heat transfer by introducing a fraction of the argon gasemployed for operation of the sputter deposition source directly intothe space between the heat exchange means and the wafer.

However, these prior uses of a gas as a heat transfer conductor havebeen limited to use of the gas in a separate chamber or compartment ofthe apparatus isolated from the portion of the process apparatusgenerating the heat, e.g., the sputter deposition chamber, eitherbecause a different gas was used for the heat transfer than the gas usedin the process, or because it is desired to maintain a higher vacuum inthe main process apparatus than that of the cooling chamber. To maintainthis separation and pressure differential, the wafer is usually clampedand/or sealed to provide this higher pressure on the backside of thewafer.

For example, in the aforementioned King patent, the pressure or vacuumin the ion implantation process chamber is said to be 7×10⁻⁷ Torr whilethe pressure behind the wafer is said to vary from 0.5 to 2.0 Torr. Inthe aforementioned Lamont patent the argon gas is said to be admittedinto the heating station at pressures of 100 to 1000 microns, whichpressures are stated to be one to two orders of magnitude higher thanthe normal argon pressure of 10 microns in the main chamber.

While it has been thought that the use of such high pressures (relativeto the processing pressures) are necessary to achieve the desired heattransfer through the gas, quite surprisingly, it has been found thatgood heat transfer can be obtained or achieved at the same pressure orvacuum conditions used in carrying out the plasma etching process, thusmaking unnecessary the prior art practice of using high pressures andsealing between the wafer and the processing chamber.

SUMMARY OF THE INVENTION

In accordance with the invention, it has been found that good heattransfer can be obtained or achieved at the same pressures or vacuumutilized for the plasma etching process or other semiconductormanufacturing processes by using certain gases for cooling or heatingwhich can also function as components of the process gas.

It is, therefore, an object of the present invention to provide aprocess and apparatus for cooling or heating a semiconductor wafer in asemiconductor processing apparatus utilizing one or more components ofthe process gas as a gas conduction heat transfer gas by directing atleast a portion of such one or more components of the process gas intocontact with a surface of the wafer and then permitting the one or moreprocess gas components to pass into the reaction chamber of theapparatus.

It is another object of the present invention to provide a process forcooling or heating a semiconductor wafer in an apparatus utilizing oneor more components of the process gas as a gas conduction heat transfergas by directing at least a portion of such one or more process gascomponents into contact with a surface of the wafer as such one or moreprocess gas components pass into the apparatus while controlling theflow of process gas into the chamber to keep the total flow of processgas into the chamber constant and while monitoring the pressure orvacuum in the apparatus to maintain the desired pressure therein.

These and other objects of the invention will be apparent from thefollowing description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow sheet of the process of the invention; and

FIG. 2 is a fragmentary cross-sectional view of a portion of a plasmaetching apparatus showing the admission of some of the process gas intothe chamber via prior contact with the surface of the wafer andmonitoring and flow control means which may be used to control the flowof process gas through the chamber to maintain the desired vacuum in thechamber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An aspect of the present invention comprises a method and an apparatusfor cooling a semiconductor wafer in a plasma etching apparatusutilizing one or more components of the process gas used in the plasmaetching process as a gas conduction heat transfer gas by directing atleast a portion of such one or more components of the process gas intocontact with a surface of the wafer as such one or more process gascomponents pass into the plasma etching apparatus. In a preferredembodiment, the flow of process gas into the chamber is controlled tokeep the total flow of process gas into the chamber constant and thepressure or vacuum in the plasma etching apparatus is monitored tomaintain the desired vacuum in the plasma etching apparatus.

As shown in FIG. 2, a plasma etching apparatus or chamber 2 is generallyindicated comprising a chamber wall 6, an evacuation port 8 which isconnected through pipe 10 and throttle valve 12 to an external pumpingsource (not shown) to maintain the desired vacuum in chamber 2, and atleast one interlock 4 through which a wafer may be introduced intochamber 2. Within chamber 2 is an rf source 20 connected to an externalpower supply (not shown) to generate the plasma and a wafer supportpedestal 30 upon which a wafer 50 is positioned for etching by theplasma generated by rf source 20. An inlet port 14 is also provided inchamber wall 6 for admission of process gas into plasma etching chamber2 through pipe 16 and flow control valve 18 from an external process gassource 40.

In accordance with the invention, means are provided in wafer supportpedestal 30 for the direction of at least a portion of one or morecomponents of the process gas into contact with the rear surface ofwafer 50 resting on pedestal 30 comprising, in the illustratedembodiment, a central bore 32 in pedestal 30. Central bore 32 is, inturn, connected through pipe 36 and flow control valve 38 to externalprocess gas source 40, which may comprise one or more components of theprocess gas used in the plasma etching process carried out in chamber 2.

The term "process gas", as used herein, is intended to define a mixtureof gases which may either function as active etching components of theprocess gas or as carrier gases. Examples of components usually presentin such plasma etching process gases are 1-4 carbon fluorinatedhydrocarbons, such as CHF₃, CF₄, and C₂ F₆ ; oxygen, NF₃, and SiF₄ ; aswell as carrier gases such as helium, nitrogen, and argon.

It should be noted, in this regard, that while process gas source 40 isshown as a single source, it will be understood that process gas source40 may comprise individually controlled sources of each of the gaseswhich collectively make up the process gas. Process gas source 40 may,therefore, include means to separately control the ratio of thesecomponents flowing through pipe 36 and flow control valve 38 to bore 32and the rear surface of wafer 50 from the ratio of such gases flowinginto plasma etching chamber 2 through inlet port 14.

Not all of these components of the process gas will functionsatisfactorily in the process of the present invention as satisfactoryheat transfer agents to conduct heat from wafer 50 to pedestal 30 at theoperating pressure or vacuum utilized in plasma etching apparatus 2.Therefore, the term "one or more components of the process gas" as usedherein to describe the portion of the process gas which is brought intocontact with the rear surface of wafer 50 through bore 32, is intendedto define only those components of the process gas which will providesatisfactory heat transfer between wafer 50 and pedestal 30. Suchcomponents include argon and the aforementioned 1-4 carbon fluorinatedhydrocarbons, such as CHF₃, C₂ F₆, and CF₄.

The total amount of such process gas components which enters chamber 2through bore 30 may be adjusted through flow control valve 38 tocomprise only about 1-5%, preferably about 2% of the total amount ofprocess gas which flows into chamber 2, with the balance flowing intochamber 2 via inlet port 14.

It should be noted that while central bore 32 is shown as the sole meansfor bringing such one or more components of the processing gas intocontact with the rear surface of wafer 50, more than one such bore couldbe provided in pedestal 30 or at least the top surface of wafer supportpedestal 30 could comprise a porous metal to permit exit of suchprocessing gas components into contact with the rear surface of thewafer 50 at more than one point. In any event, it will be noted thatwhile wafer 50 is shown resting on pedestal 30, there is no seal orclamping type retention provided between wafer 50 and pedestal 30 to tryto confine or restrict the movement into plasma etching chamber 2 ofmolecules of the one or more components of the process gas which havebeen brought into contact with the rear surface of wafer 50 through bore32 in pedestal 30. Such sealing means are unnecessary in the practice ofthe present invention due both to an absence of pressure differentialsbetween chamber 2 and the rear surface of wafer 50 as well as the use ofone or more beat transfer gases which are also components of the processgas used in the plasma etching process, i.e., that heat transfer gasesand the plasma etching gases are both used at the same pressure and theheat transfer gases comprise a portion of the plasma etching gases.

Pressure or vacuum monitor means 60 are also provided within plasmaetching chamber 2 to monitor the total gas pressure in chamber 2. Asignal from gas monitor means 60 may be fed to throttle valve 12 whenthe pressure exceeds a predetermined amount so that chamber 2 may befurther evacuated. Alternatively or supplementally, the pressure orvacuum in chamber 2 could be controlled by controlling the flow ofprocess gas entering chamber 2 through flow control valve 18 usingcontrol means 70.

When the pressure or vacuum in chamber 2 is controlled by throttle valve12 through control means 70, the flow of process gas through inlet valve18 and pipe 16 from process gas source 40 may be made constant byinitial adjustment of control valves 18 and 38.

Plasma etch chamber 2 is maintained at a pressure of from about 40 to200 millitorr, preferably about 60 millitorr. The total flow of processgas into chamber 2 which is necessary to maintain the plasma in chamber2 ranges from about 130 to about 300 sccm. Of this amount, from at leastabout 0.05 sccm, preferably about 0.15 sccm, flows through bore 32 inpedestal 30 with the balance entering chamber 2 through process gasinlet port 14.

It will be noted that the silicone rubber pad conventionally placedbetween wafer 50 and pedestal 30 has been eliminated since the pad isnot needed for conduction of heat from wafer 50 and pedestal 30.Instead, pedestal 30, which may be formed of aluminum metal, may beanodized to provide from about 0.5 mils to about 2.0 mils of aluminumoxide as electrical insulation between wafer 50 and pedestal 30 withoutmaterially interfering with the heat flow from wafer 50 to pedestal 30through the process gas molecules in accordance with the invention.

To illustrate the advantages of the invention, the temperature of asilicon wafer subjected to plasma etching in a conventional etchingchamber having a silicone rubber heat transfer means was measured andfound to be 115° C. after a sufficient time to achieve steady stateconditions, i.e., after about 3-5 minutes of etching. In contrast, thetemperature of a silicon wafer cooled in accordance with the inventionand etched under the same etching conditions and time period was foundto be 65° C., indicating the superior heat transfer realized using theprocess and apparatus of the invention.

While the above preferred embodiment refers to the cooling ofsemiconductor wafers in plasma etching apparatus, methods and apparatusin accordance with the present invention can also advantageously heatwafers within semiconductor processing equipment. For example, in a CVDchamber a process gas can be used as a heat transfer agent by flowing itbetween a heated pedestal and a wafer supported over the pedestal andthen into the CVD chamber. Again, the process gas used for the heattransfer can comprise a chemically active gas, an inert or carrier gas,or combinations thereof. The process gas can also include any gas notdestructive to the processing of the semiconductor wafer whichaccomplishes the heat transfer of the present invention. For example,the process gas could be an inert gas not otherwise used in the processbut which can be released into the reaction chamber without degradingthe processing of the wafer.

While this invention has been discussed in terms of a single wafersystem, it is equally applicable to multiple wafer systems such as theApplied Materials 8310 etcher which processes 18 wafers simultaneously.In the case of multiple wafer etcher systems, it is still desirable tomaintain the total pressure of the process gasses to the range of130-300 sccm, and the flow of process gas against each wafer at leastabout 0.05 sccm. In the case of the 8310 etcher, this results in a totalflow of process gasses against the wafers of at least about 0.9 sccm.The total gas flow for cooling is still in the range of 1-5% of theprocess gas within the apparatus.

Thus, the invention provides an improved process and apparatus forheating or cooling a semiconductor wafer in a semiconductor processingapparatus using the process gas as the heat conducting means between thewafer surface and a heat conductor. Preferably, the flow of process gasinto the chamber is controlled to keep the total flow of process gasinto the chamber constant and the pressure in the apparatus is monitoredto maintain the desired pressure therein.

Having thus described the invention, what is claimed is:
 1. Apparatusfor cooling a semiconductor wafer in a plasma etching apparatus byutilizing one or more thermally conductive components of the process gasused in the plasma etching process as a gas conduction heat transfer gaswhich comprises:a) a support surface for supporting said wafer in saidplasma etching apparatus, said support surface further function as aheat sink; b) means for directing at least a portion of said one or morethermally conductive components of said process gas into thermal contactwith a rear surface of said wafer facing said support surface throughone or more openings in said support surface;said apparatus beingfurther characterized by the absence of clamping or sealing meansbetween said wafer and said support surface where after said one or morethermally conductive components of said process gas are directed intothermal contact with said rear surface of said wafer the gas is allowedto flow into the plasma etching chamber.
 2. The apparatus of claim 1which further comprises means for controlling the flow of process gasthrough said plasma etch apparatus to maintain a total flow of fromabout 130 to about 300 sccm of process gas through said plasma etchingapparatus.
 3. The apparatus of claim 1 which further comprises means formonitoring the vacuum in said plasma etching apparatus to maintain avacuum in said plasma etching apparatus of from about 40 to about 200millitorr.
 4. The apparatus of claim 1 wherein said support surfacesupports a plurality of said wafers at one time and wherein a portion ofsaid process gas is directed into thermal contact with a rear surface ofeach of said wafers.
 5. Apparatus for heating or cooling a semiconductorwafer being processed in a semiconductor wafer processing apparatuswhich comprises:a wafer support in said chamber comprising means forsupporting said semiconductor wafer in said chamber without clamping orsealing said wafer to said wafer support; and b) means for directing atleast a portion of one or more thermally conductive components of theprocess gas used in the reaction chamber into thermal contact with arear surface of said wafer facing said wafer support to change thetemperature of said wafer and thereafter allowing said one or morethermally conductive components of the process gas into the reactionchamber.
 6. The apparatus of claim 5 wherein said means for directing atleast a portion of said one or more components of said process gas intocontact with said rear surface of said wafer further comprise one ormore openings in said wafer support facing said rear surface of saidwafer.
 7. The apparatus of claim 5 wherein a plurality of said wafersupports is present in said apparatus and wherein a portion of saidprocess gas is directed into thermal contact with a rear surface of eachof said wafers.